Varieties of photoelectric conversion devices using organic compounds as functional materials have been studied in order to develop an inexpensive and safe photoelectric conversion device, because it is extremely difficult to develop such an inexpensive and safe photoelectric device using a single crystalline silicon, a polycrystalline silicon or an amorphous silicon.
A photoelectric conversion device is a device for converting light energy to electrical energy (voltage.times.current). Therefore such a photoelectric conversion device is evaluated mostly with respect to its energy conversion efficiency. The presence of an inner electric field is necessary to generate a photo current. As devices for generating the photo current, MIS-type devices, hetero-pn-junction type devices comprising an n-type inorganic semiconductor and a p-type organic semiconductor, and hetero-pn-junction type devices comprising different types of organic semiconductors are known. It has been reported that the hetero-junction type devices comprising different types of organic semiconductors attain the highest energy conversion efficiency of the above photoelectric conversion devices.
As one group of organic compounds which form the junction, condensed polycyclic aromatic compounds, for instance, dyes such as Malachite Green, Methyl Violet and pyrylium compounds, flavanthrone, and perylene pigments are reported, and as another group of organic compounds which form the junction, phthalocyanine pigments and merocyanine dyes have been reported. Of the photoelectric conversion devices using these compounds, it is reported that a photoelectric conversion device with a structure of ITO/copper phthalocyanine/perylene pigment/Ag attains an energy conversion efficiency of 0.95% with AM-2 radiation at 75 mW/cm.sup.2, which is the highest energy conversion efficiency (Voc=0.45 V, Jsc=2.3 mA/cm.sup.2, ff=0.65) of those attained by devices using organic compounds (C. W. Tang Appl. Phys. Lett, 48, 183 (1986)). Japanese Patent Publication 62-4871 discloses a photoelectric conversion device with the same structure as that of the above photoelectric conversion device except that the perylene pigment employed therein is replaced by another perylene pigment, which attains an energy conversion efficiency of 1% (Voc=0.44 V, Jsc=3.0 mA/cm.sup.2, ff=0.6).
Photoelectric conversion devices comprising organic compound layers have the advantages over other photoelectric conversion devices that illumination can be carried out through a transparent electrode and photo charges can be generated by two kinds of organic materials, so that the spectral sensitivity range can be broadened. The above-mentioned report by C. W. Tang indicates that electric charges are produced in a perylene pigment by the application of light with a wavelength of 450 to 550 nm, and in the copper phthalocyanine by the application of light with a wavelength of 550 to 700 nm. Further, the ff value of the above photoelectric conversion device is larger than those of other photoelectric conversion devices. It is considered from this fact that the inner electric field produced therein is also larger than those produced by other photoelectric conversion devices. In this sense, the photoelectric conversion device by Tang appears to be the most suitable one yet developed for use in practice. However, it is, in fact, still unsatisfactory for use in practice since the generation of the photo current must be further improved for practical use.
It is known that the photo current value can be increased by making the organic material layers in the photoelectric conversion device thinner. This is because the bulk resistances of the organic material layers are so large that an organic material layer which actually contributes to the generation of photo current, that is, the so-called photo active layer, must be made extremely thin in order to obtain a sufficient photo current for use in practice. However, when the organic material layer is made thin, pin holes are formed in the organic material layer and a short-circuit occurs within the device. Therefore the method of reducing the thickness of the organic material layer has not yet been made sufficiently practical in view of the yield of the photoelectric conversion device.